The present invention relates to a device for the conveying and releasable fixing of substrates to be printed.
A conveyor and fixing unit are provided in particular for fitting in a printing machine which is to be used to apply solder paste to a substrate in the form, for example, of a printed circuit board, at predetermined points (pads). The substrate may also be a ceramic object, a flexible tape, a wafer or any other body suitable for the material application. Besides solder paste, the material to be applied may also be an adhesive, silicone or epoxy resin. In order to execute a printing process for the application of, for example, solder paste, the substrate to be printed is conveyed by the conveyor unit for example from a substrate stack to the printing machine, where the substrate is aligned relative to a mask provided there, and solder paste is then applied to the areas of the printed circuit board to be printed by means of a doctor blade.
During the printing process it is necessary for the substrate to be fixed, in order to prevent it from slipping from the position In which it is aligned relative to the mask, since this would lead to an undesired printed image, as for example a wetting of pad intermediate spaces with solder paste or insufficient coverage of the pads with solder paste.
For timed and localized fixing of the substrate it is already known that the substrate may be fixed by a clamping process at the substrate side edges, wherein the substrate is clamped by a movable guide cheek against a stationary guide cheek. This clamping leads to a clamping pressure force in the plane of the substrate, referred to below as the fixing plane.
During the actual printing process the substrate is supported from below by supports, since the doctor blade exerts a compressive force on the substrate. This compressive force leads to warpage of the substrate from the fixing plane defined above, since there is no support in the edge zone (conveyance zone). Because of this lack of support, thin substrates in particular are unable to absorb these doctor blade forces in the edge zone, leading to downwards movement of the substrate and leading in turn to the upwards warpage of the substrate in the center described above. Due to the bending of the substrate, positioning inaccuracies occur over the surface of the substrate between the substrate and the mask, or the substrate top to be printed and the mask bottom, so that the desired coverage of the pads on the substrate with solder paste does not occur. The problem described above occurs especially with very thin substrates, which have an increased tendency to bend.
It is an aspect of the present invention to eliminate the problem described, by creating a conveyor and fixing unit for a printing machine which avoids bending of the substrate and thereby facilitates the most even possible coverage of the pads over the whole of the substrate surface to be printed.
The invention creates a unit for the conveyance and releasable fixing of substrates to be printed during a printing process, wherein the unit has a conveyor device which conveys the substrate to a fixing device which in turn fixes the substrate by means of contact with side edge areas of the substrate arranged in the direction of conveyance, and has support means on which the substrate lies in the area of the side edges. This enables the substrate to lie on the support means during fixing and printing, and specifically in the area of the edge areas of the substrate which are at the side in the direction of conveyance, so that the substrate in the form for example of a printed circuit board is supported from below, thereby avoiding warpage of the printed circuit board.
Consequently when the printed circuit board is releasably fixed for the printing process by the conveyor and fixing unit according to the invention, then the printed circuit board is supported not only in its center, i.e. within the side edges, but also directly at the side edges, thereby avoiding any downwards bending of the side edges relative to the longitudinal center axis of the printed circuit board in the direction of conveyance in the side edge area. The device according to the invention therefore provides a largely plane-parallel alignment of the printed circuit board in the plane in which the doctor blade is guided over the mask during the printing process, resulting in an even coverage of the printed circuit board pads with solder paste.
According to an advantageous development of the invention it is provided that the support means are in the form of tongue-shaped support elements spaced apart, which support from below the side edge areas running in the direction of conveyance of the printed circuit board.
The tongue-shaped support elements may be an integral part of a support bar, with conveyor rollers in the area between the individual support elements on which the substrate may be conveyed into, and also away from, the area of the fixing device. By this means the substrate to be printed may be placed via a feeder device on to the conveyor device, which then conveys the substrate into the area of the support bar with the tongue-shaped supports, which have conveyor rollers arranged between them. The substrate is then fixed for the printing process by means of the fixing device so that the top of the printed circuit board to be printed with the pads to be printed is aligned with the mask cut-outs, resulting in the printable substrate top side lying plane-parallel on the mask underside.
If the inventive conveyor and fixing unit is used to hold very thin substrates in place for the printing process, then it is also possible to provide the fixing device with hold-down strips which act on the side edge areas of the substrate from above, pressing them against the support elements so that the thin substrate is pressed against the support elements, thereby providing additional fixing. For good contact, these hold-down strips are therefore able to give additional reinforcement to the linear contact between the bottom of the substrate and the top of the support elements.
The substrate is therefore conveyed by the conveyor device into the area of the fixing device which, according to an embodiment of the invention, may be lowered relative to the conveyor device, with the support elements and the hold-down strips moving together to clamp the substrate during the lowering process. The lowering of the fixing device thus causes the support elements and the hold-down strips to move towards one another so as to clamp the substrate between them in the manner according to the invention.
The conveyor rollers already referred to above are able to move the substrate, via a drive provided on the fixing device, into the intended clamping position after handover of the substrate from the conveyor device. On completion of the printing process they are able to transfer the substrate from the area of the fixing device back to the conveyor device for movement away from the area of coverage of the printing machine. This enables a printing machine provided with the device according to the invention to automatically accept and print substrates and to convey them onwards.
For this purpose the conveyor device may have conveyor belts on which the substrate lies during conveyance and by which the substrate is conveyed into the area of the fixing device, from which it is removed after the printing process has taken place.